1. Field of the Invention
The present invention concerns plates for defibering or refining paper pulp, having an abrasion resistant comminuting surface for breaking up clumps of fibers.
2. Description of the Prior Art
In paper pulp defibering or refining machines such as those described in U.S. Pat. Nos. 3,117,603 and 4,423,845, for example, two plates rotating relative to each other each carry an assembly of circular sector shaped plates disposed in a circular ring, the operative surfaces of the plates having generally radial grooves and being made from an abrasive material such as alumina, zirconium oxide or silicon carbide, for example. The comminuting surface thus has a large number of ridges separated by grooves, the shape of the ridges and grooves being carefully chosen to defiber the paper pulp. The material inserted between the plates flows in the radial grooves and the shape, spacing and direction of the grooves determine the size of the paper pulp fibers obtained. The dimensions and the position of the plate must therefore be highly accurate.
The refining and defibering plates usually employed at present are made either from cast iron alloys known as NIHARD cast irons or from stainless steel.
The ridges on the plates tend to wear down with use, due to the abrasive effect of the paper pulp fibers or abrasive materials that the paper pulp contains. The resulting progressive deformation of the ridges renders the plate unusable after a period which is usually not very long.
Cast iron alloy plates are manufactured by casting. When worn down they have to be discarded.
Stainless steel plates wear more quickly, but they can be recut when worn to restore the initial profile of the grooves. This can be done once or twice. However, recutting tends to weaken the plate and is a costly operation.
One proposal for extending the life of paper pulp refining or defibering plates is to exploit the advantageous properties of harder materials such as grains of tungsten carbide.
For example, document EP-A-0 480 851 proposes to produce an abrasion resistant surface comminuting or refining plate by infiltration of a molten brazing alloy into a heap of grains of tungsten carbide in a mold. This technique requires a mold and therefore cannot be used for resurfacing an existing comminuting or refining plate.
Document WO-A-91 02841 proposes the application of an abrasive surface layer to a refining plate by using a laser beam to melt the surface layer of the steel substrate and spraying onto the molten surface layer an abrasive material such as tungsten carbide in the form of powder with a particle size between 120 microns and 2.5 millimeters. The resulting layer is abrasive and necessarily thin because only one melting and spraying pass is possible. This treatment tends to weaken the substrate, by reducing the thickness of its homogeneous metal part. Also, the surface layer does not have good abrasion resistant properties.
Document WO-A-92 11941 proposes the manufacture of a refining plate with an abrasion resistant layer by cold isostatic pressing of a mixture of tungsten carbide powder and a powdered alloy of iron, nickel, chromium, carbon and molybdenum. The tungsten carbide content is low, being between 10% and 50% by weight of the mixture of powders. This technique requires a mold and a costly high-pressure press and is thus not applicable to resurfacing an existing defibering or refining plate.
The problem to which the present invention is addressed is that of designing a new paper pulp defibering or refining plate structure and a method of manufacturing it, the plate having improved abrasion resistance, without reducing the mechanical strength of the body of the plate, and a greater efficiency in use.
In particular, the invention is directed to increasing the abrasion resistance of the tops of the ridges, which constitute the most heavily loaded areas of the plate, whilst favoring flow of paper pulp in the grooves.
To this end, the basic idea of the invention is to resurface the top of the ridges selectively using an appropriate abrasion resistant material, retaining for the base of the ridges and the bottom of the grooves a material which favors flow of paper pulp. Flow is improved by the polishing effect of the flow of paper pulp.
The problem is then how to carry out this resurfacing at reasonable cost, with good homogeneity and good adherence to the body of the plate, using a resurfacing material containing a high proportion of hard abrasion resistant material, with a sufficient resurfacing thickness, and without distorting the body of the plate, in order to preserve the accuracy of the geometrical shape of the plate.
The solution of the invention is to resurface only the tops of the ridges of the plates by means of a laser beam in association with a powder injector.